This invention relates to a module for generating a clock signal for the upstream channel of a bidrectional point-to-multipoint network, to a center of the network, and to a method of generating the clock signal for the upstream channel of the bidirectional point-to-multipoint network.
As point-to-multipoint networks in the area of wireline networks, hybrid fiber/coax networks, for example, are known. In the area of wireless networks, radio networks, for example, particularly cellular radio networks or cellular mobile radio networks, are known. In the area of mixed structure networks, hybrid fiber/radio networks, for example, are known.
In point-to-multipoint networks, information is transmitted from a center over a downstream channel to a plurality of transmitting/receiving facilities. Such transmitting/receiving facilities are, for example, decoders, set-top boxes, mobile telephones. The information contains, for example, television signals, video signals, Internet signals, telephone signals. Over an upstream channel, information is transmitted from the transmitting/receiving facilities to the center. The information transmitted in the upstream channel contains, for example, request signals for videos, Internet access signals, telephone signals.
Access by the transmitting/receiving facilities to the upstream channel must be controlled. Several access techniques are known, including time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA). A special version of CDMA is synchronous CDMA (S-CDMA).
In CDMA, each transmitting/receiving facility is assigned an individual code. The transmission of the data of all transmitting/receiving facilities takes place in the same frequency range using spread spectrum modulation. In S-CDMA, the transmission of the codes must additionally be coordinated in such a way that the codes arriving at the center are synchronous.
The invention provides a module for generating a clock signal for the upstream channel of a bidirectional point-to-multipoint network comprising a center and a plurality of transmitting/receiving facilities, with the upstream channel being accessed using S-CDMA in particular, wherein a reference clock is provided for generating a reference clock signal, wherein the reference clock is connected to a reference clock control and a numerically controlled oscillator, the reference clock control being adapted to compare the reference clock signal with digital master clock frequency information received over the network, and to derive from the result of the comparison a control signal for adjusting the reference clock signal to the master clock signal, and wherein the numerically controlled oscillator is adapted to generate the clock signal for the upstream channel from the reference clock signal and individual, digital phase correction information.
The invention further resides in a center of such a network, the center comprising a transmitting unit for transmitting information over the network, a clock generator for generating a master clock signal, and a receiving unit, controlled by the master clock signal, for receiving information, wherein the receiving unit is adapted to derive from the received information digital phase correction information for individual transmitting/receiving facilities and to feed said phase correction information to the transmitting unit, the transmitting unit being adapted to transmit the master clock signal in the form of digitally encoded frequency information and the individual phase correction information together with data over the network.
Finally, the invention resides in a method of generating such a clock signal wherein a master clock signal generated at the center is transmitted in the form of digitally encoded frequency information over the network to the transmitting/receiving facilities, the transmitting/receiving facilities deriving the clock signal for the upstream channel from the digitally transmitted frequency information and a local clock signal, the center comparing the phase of the clock signal received from each transmitting/receiving facility with the phase of the master clock signal and deriving from the result of this comparison digital phase correction information individual to each transmitting/receiving facility, which it transmits to the respective transmitting/receiving facility (3), and the respective transmitting/receiving facility (3) adjusting the clock phase for the upstream signal using the individual phase correction information.
Further advantageous features of the invention are defined in the dependent claims.